It is common to design agricultural vehicles, such as tractors, with so-called structural engines. In such vehicles, the engine, together with the transmission and the rear axle, constitute the rigid chassis of the vehicle. Because there is no suspension between the chassis and the ground, at least at the rear of the vehicle, shocks resulting from unevenness in the terrain are transmitted directly to the driver's cab.
To improve the comfort of the driver, it is known to pivot the cab about a transverse axis at its front end and to provide at least one suspension unit, comprising a spring and a damper, between the rear end of the cab and the chassis. This allows a cushioned up and down movement of the cab on the chassis. Such an arrangement is shown in FIGS. 1 and 2 of the accompanying drawings, in which FIG. 1 is a schematic representation of a cab that is pivotable at its front end and supported at its rear end on suspension units, and FIG. 2 is a detailed perspective view of the suspension unit in FIG. 1.
In FIG. 1, the front end of the cab 10 is pivoted about an axis 12 that is fixed to the chassis. A suspension unit 14 at the rear of the cab, as better shown in FIG. 2, has a flexible strap 16 attached to a bracket 18 fixed to the vehicle chassis at one end and at its other end to a bracket 20 attached to the cab 10. The flexible strap 16 acts as an anchor which allows the cab 10 to move up and down relative to the chassis while preventing it from moving upwards beyond acceptable limits.
The suspension unit 14 is shown as having a strut 22 that incorporates a spring 24 and a damper 26. The spring 24 which supports the weight of the cab, need not be a coil spring, nor need it be concentric with the damper. It may alternatively be a gas spring or a leaf spring mounted separately from the damper. FIG. 2 also shows that a resilient bump stop 28 is provided to cushion the cab if the suspension ever reaches the end of its permissible travel.
While such a cab suspension goes some way towards improving the ride quality in the cab, it still suffers from the disadvantage that when the tractor is being driven along an incline, the cab and the driver's seat, though parallel to the ground, are inclined relative to the horizontal and this causes discomfort to the driver, aside from being disconcerting. Similarly, when the tractor is being driven up or down an incline the cab can assume an extra-pitch angle causing discomfort to the driver.
It is therefore desirable to provide the cab of an agricultural vehicle with a self-levelling suspension system which maintains an optimal attitude of the cab controlling roll and pitch angles, even if the inclination of the ground over which the vehicle is travelling varies within certain limits.
In U.S. Pat. No. 6,273,203, there is disclosed a suspension system that employs four hydraulic actuators capable of tilting the cab about mutually inclined axes and a control system for independently controlling each of the hydraulic actuators in dependence upon signals received from sensors, which can be constructed as inclinometers or gyroscopes. In this known system, the fact that all four actuators can be independently controlled makes for a complicated control system. This is because the control algorithm needs to take into account when an actuator is near the bottom or top of its stroke and is therefore unable to bring about the required tilt of the cab.
The Applicants' earlier U.S. Pat. No. 7,198,125 mitigates some of the above disadvantages and provides a vehicle having a chassis and a cab connected to the chassis by means of a support system which comprises two pairs of hydraulic actuators, each pair of actuators being operative to tilt the cab relative to the chassis about a respective one of two mutually inclined axes. The two actuators of each pair are connected to a common pumping element in such a manner that whenever the volume of hydraulic fluid in one of the actuators in a pair is reduced, the volume of hydraulic fluid in the other actuator of the same pair is correspondingly increased.
A disadvantage of U.S. Pat. No. 6,273,203, U.S. Pat. No. 7,198,125 and other prior art cab suspension systems which use hydraulic dampers as actuators to level the cab stems from the fact that they require a dedicated pumping unit. Such pumping units need furthermore to be fairly substantial as they are required to supply hydraulic fluid to the actuators sufficiently rapidly to counteract changes in the inclination of the chassis.